This invention generally relates to multi-chambered thermal oxidizers that employ a rotary valve. More specifically, the invention relates to sealing apparatus used with such devices for preventing the unwanted mixing of treated and untreated gases.
Thermal oxidizers such as regenerative thermal oxidizers (RTOs) are known (see, e.g., U.S. Pat. Nos. 5,026,277 and 5,352,115), and are used for oxidizing pollutants, such as hydrocarbon vapors in air. In a conventional "reverse flow" RTO, a pollutant-laden gas to be cleaned is directed through a first "hot" heat exchanger and then into a combustion chamber; the oxidized gas is then directed out of the combustion chamber and through a second "cool" heat exchanger. This flow through the heat exchanging beds is then reversed. By alternating the flow of cool gas to be cleaned through the "hot" heat exchanger, and moving the hot cleaned gas from the combustion chamber and through the "cool" heat exchanger, an RTO can continuously operate to efficiently oxidize pollutant-laden gases.
Manufacturers always seek higher destruction efficiencies for thermal oxidizers, while balancing the costs toward obtaining this goal against fabrication and use costs. The destruction efficiency of an RTO is dependent, in part, on the effectiveness of the valve seals in preventing the untreated, pollutant-laden process stream from exiting the exhaust stack without first passing through the combustion chamber. With conventional, multi-bed RTOs, multiple poppet and butterfly valves and/or diverter dampers are used, which typically employ either a tadpole or metal-to-metal seat.
Rotary valve, "single-can" RTOs have also become commercially available. These are multi-chamber thermal oxidizers that employ a rotary valve assembly, as shown in U.S. Pat. Nos. 4,280,416 to Edgerton, 5,016,547 to Salem and 5,460,789 to Eisenmann. Rotary valve RTOs have several advantages over conventional RTOs. They can be made more economically, and do not require multiple butterfly or poppet valves. Also, rotary valve RTOs provide a smoother flow of the process gas through the device than conventional RTOs, since there are lower pressure fluctuations in the process duct work as a result of the absence of abrupt flow reversals.
Rotary valve RTOs present a new and different sealing problem, however, since sealing must be accomplished across the interface of the rotating and fixed valve portions, to avoid unwanted mixing of the untreated and treated gas streams. Rotary valve RTOs have used either a metallic wiper seal and/or a close-tolerance (machined) gap seal to minimize leakage between the fixed and rotating portions of the device, with or without the use of "sealing" or "purge" gas. There are disadvantages to each of these approaches.
With a "wiper" seal design, a wear surface is placed on a fixed portion of the valve while the rotating section is allowed to make sliding contact with the wear surface. However, the wear surface eventually needs to be replaced. Also, the nature of the valve design and the placement of flow passages make it difficult to access the inner sealing surfaces for repair or replacement.
A close-tolerance seal may also be used, but it is difficult and expensive to fabricate, particularly with the large valves used in RTO applications, which may be 5-12 feet in diameter. Since a precisely-machined valve seal is difficult to achieve, past rotary valve devices have also used a volume of clean, pressurized "sealing" or "purge" gas to minimize cross-contamination.
As used here, "purge" gas refers to a clean gas used to displace untreated vapors in the oxidizer bed, while "sealing" gas refers to a separating clean air curtain provided at a higher pressure than the process stream, to prevent one gas from mixing with another adjacent gas. As used here, purge gas under either negative (vacuum) or positive pressure may be employed.
As discussed below, a primary advantage of the present invention is realized through the use of a "sand seal". Sand seals have long been used in kiln applications as a draft seal. However, it is not believed known to use a sand seal in connection with a rotary valve thermal oxidizer.
Accordingly, it would be advantageous to provide a more efficient yet economical rotary valve thermal oxidizer that employs superior valve sealing devices.